By establishing a temperature differential between two heat sinks which interface with a device categorized as a heat engine or its equivalent, it is possible to derive mechanical or electric output power, subject to a limit set by the efficiency of the perfect Carnot cycle.
Boltzmann derived the Stefan-Boltzmann law, by which the rate of heat radiation is proportional to the fourth power of absolute temperature, by deducing the connection between temperature and the energy density of black-body radiation. This involved the conception of an `aether engine`, a Carnot engine without any working substance, driven by the pressure of radiation.
In principle, radiation admitted to a cylinder expands to drive a piston and so does mechanical work as the temperature of the radiation reduces. The temperature reduction is argued on the basis that if work is done and removed as useful output then radiant energy has to flow in to replenish the system at the end of the cycle and this could only occur if the temperature had, in fact, reduced.
Such argument derives from the second law of thermodynamics, but one wonders about that reference to the temperature of radiant energy. We have come to accept that radiation comprises photons having a spectrum of frequencies and do not regard photons as having a temperature. Only radiating matter can be said to have a temperature, usually related to the thermal energy of its molecules via the Boltzmann constant.
Thus radiation has an intensity characteristic of its energy concentration and it has a quality representing its source by virtue of its frequency spectrum. Blackbody radiation from sources of lower temperature contains photons having frequencies which are the same as those from higher temperature sources. It is the distribution of energy as between these different frequencies or the number of photons at a particular frequency which characterizes the temperature of the source. Therefore, if that radiation from the cooler source can be concentrated in some way it can heat an absorbing surface to a higher temperature.
The formal statement of the second law of thermodynamics is very carefully worded to make it clear that heat cannot travel from a cooler body to a warmer body of its own accord, but the intervention of a means for focusing photon energy, as by use of a lens or mirror, or as by the fanciful textbook notion of the intervention of the Maxwell demon can affect that self-accord.
The use of mirrors or lenses to reflect or refract thermal radiation, whether sourced in a heated or cooled surface, was a curiosity in early scientific experiments. Indeed, one such notable experiment was performed by Count Romford in Edinburgh, Scotland, in the year 1800. He repeated the experiment of Pictet, by which the radiation and reflection of cold was demonstrated, to show that objects seated at one focus of a concave mirror could be cooled by a cool object seated at the other focus. See article entitled: `Pictet's experiment: The apparent radiation and reflection of cold` by J. Evans and B. Popp, American Journal of Physics, vol. 53, p. 737 (1985).
The point about this experiment was that there is heat transfer until there is equilibrium between the radiation exchanged by the two surfaces as governed by the areas of the two surfaces put in juxtaposition by the mirror focusing. The temperatures adjust to keep the radiation in balance, unless some is absorbed and conducted away in the apparatus.
From a technological viewpoint these phenomena are traditionally deemed to be of little consequence, though they do find application in the design of bolometers.
So far as this inventor has been able to ascertain, it has not been foreseen in the prior art that, by combining optics and heat engines and focusing radiation in the manner suggested, the concentration of heat radiation driven at the speed of light can develop temperatures at an absorbing surface which exceed those of the radiating source and can be used to convert heat into engine power. Nor, so far as the inventor is aware, has it been suggested that useful power could be generated by combining a heat engine and an optical system to concentrate heat radiation sourced in a radiating surface which is within the fabricated structure of the converter.
It has been suggested to combine a heat engine and a parabolic mirror, with the flow of heated fluid used to power the engine passing through a tubular heat exchange element at the linear focus of the mirror. Such an arrangement for caputuring solar radiation is disclosed in `Solar Electric Systems`, Hemisphere Publishing Corporation, USA (1984), Editor George Warfield. See paper by Jean-Pierre Causse entitled `Solar Thermal Power Plants` at pp. 101-113. Also, the paper by Jerald D. Parker entitled `Components of Solar Thermal Electric Systems` at pp. 89-100 is relevant because it suggests the use of a Stirling engine. However, these specific proposals relate to solar power, that is heat energy sourced in the sun at a temperature of 6,000K. There is no teaching in these prior art disclosures suggesting that the heat engine can be driven by radiation sourced at a temperature that is less than that of the input to the engine.
Indeed, it would not b feasible to power a heat engine from the solar source if the proposals of the subject invention were applied to that purpose, simply because no practical heat engine can be built to operate at that 6,000K temperature.
The subject invention was the basis of a priority filing dated 16 Nov. 1989 and the inventor notes that John Maddox, the Editor of the journal Nature, has had occasion since that date to raise the subject of possible breach of the second law of thermodynamics in his editorial `Maxwell's Demon Flourishes` (Nature, vol. 345, p. 109; 1990). Also, in this same journal (Nature, vol. 346, p. 802; 1990), there is a report sourced in the Enrico Fermi Institute in Chicago announcing that terrestrial sunlight has been concentrated by a two-stage system including a mirror to an intensity which exceeds that at the surface of the sun.
It is clear, therefore, that by the astute use of mirrors or lenses, two thermally radiating surfaces at different temperatures and of different area can be caused to maintain a state of equilibrium at those temperatures, simply because the same rate of heat is radiated by each surface. The proviso is that the radiation is guided both ways through the optical system so as to be confined to exchanges restricted to those surface areas.
On this basis, since some heat energy can be drawn off by conduction from the hotter surface, one can contemplate radiant transfer of heat from the cooler body to the warmer body, notwithstanding the validity of the second law of thermodynamics as correctly worded. Here the proviso is that heat is continuously extracted from the hotter body via a separate channel and replenishment heat is continuously supplied to the cooler body also via a separate channel. Energy has, of course, to be conserved, a requirement of the first law of thermodynamics.